The living environment: Marine and terrestrial

2016

Given its extreme conditions, the Antarctic has a surprising diversity of ecosystems. Antarctica is the coldest, windiest, driest and highest continent. Only about 0.18 per cent of the continent is ice-free. Since plants, the invertebrates associated with them and most seabirds require bare rock as growth or breeding habitats, the ice-free areas are important for their survival. The limited availability of ice-free areas means that many species are often found breeding close to each other.

Because Antarctica and the Southern Ocean are distinct in their physical properties from other parts of the world, they have many endemic species. For example, 100 per cent of the nematodes, 50 per cent of the lichens and more than 30 per cent of the terrestrial invertebrates on the Antarctic continent are found nowhere else (Pugh & Convey 2008), and the dominant fish species in the Southern Ocean are endemic to the region (Cheng et al. 2003).

Marine environments

The climate of the Southern Ocean and its circulation play a central part in the global carbon cycle (IPCC 2013). The Antarctic marine ecosystem, comprising the area south of the Antarctic Polar Frontal Zone, is a productive area that many species—such as seals, whales and seabirds—depend on. It also supports valuable commercial fisheries. Antarctic krill is the keystone species in the Antarctic marine ecosystem (Kawaguchi & Nicol 2014). It is expected that krill, together with other creatures at the lower trophic levels, will experience the greatest effects of a warming ocean (Constable et al. 2016). Although metabolic rates of such animals may initially increase, if warming continues, this effect is likely to be reversed and lead to a decrease in production because of a lack of available nutrients or increased predation by higher trophic levels. Conditions in the Southern Ocean are changing at a rate that exceeds the global average. But it is difficult to assess possible future states, because current climate models do not provide good simulations of the events in the southern regions (Constable et al. 2016).

The pelagic environment

Marine microorganisms form the basis of Antarctic food webs. They include vast numbers of bacteria, phytoplankton (single-celled plants) and zooplankton (single-celled animals), and comprise around 90 per cent of the living matter produced in Antarctic water. Bacterial communities occur throughout the water column of the Southern Ocean, as well as in the sea ice. These tiny organisms provide food for zooplankton, krill, and fish and other vertebrates. The biomass of phytoplankton is estimated to be 5 billion tonnes; there are also about 1.2 billion tonnes of bacteria and some 0.6 billion tonnes of protozoans (Marchant 2002). The number of species for many groups of organisms is still unknown. The Census of Antarctic Marine Life found many new species that are still being identified; this is particularly true for bacteria (Gutt et al. 2010). The phytoplankton (diatoms, dinoflagellates, cilliates and other protists) in the Southern Ocean comprise 560 known species (Scott & Marchant 2005), but only a few are widely dominant, and their community structure is not constant throughout the Southern Ocean (Ishikawa et al. 2002). One group, the diatoms, are responsible for most of the primary production. Their level of productivity varies greatly with season; it is highest in spring and early summer (Westwood et al. 2010). Most of their production is consumed or recycled by bacteria and protozoa (Becquevort et al. 2000).

Intense phytoplankton blooms occur in Antarctic waters during spring and summer, when increasing sunlight melts the sea ice and warms the ocean. The high light conditions and high nutrient content in the surface waters are ideal conditions for the growth of phytoplankton. During photosynthesis, phytoplankton take up CO2 that dissolves in the ocean from the atmosphere. They also produce dimethyl sulfide, a natural aerosol, which is released into the atmosphere. Here, it helps cloud formation, because it acts as a cloud condensation nucleus (Woodhouse et al. 2010), and increases the reflectance of solar radiation from Earth. Thus, these single-celled organisms not only support the food web but also influence the biochemistry of the ocean, and play a vital role in affecting global climate by reducing CO2 in the atmosphere and altering global heat balance. In turn, they are affected by anthropogenic changes to the atmosphere. Ozone depletion has increased the damage phytoplankton experience from increased ultraviolet B radiation. Anthropogenic environmental changes are likely to have far-reaching impacts on the Antarctic marine ecosystem.

A keystone species in the Southern Ocean ecosystems is krill—small shrimp-like crustaceans that rank highly on the menu of many top predators, such as fish, whales, seals, penguins and flying seabirds. Krill feed on phytoplankton and sometimes small zooplankton. Copepods (minute crustaceans) also graze on phytoplankton, making up most of the biomass in many pelagic zooplankton communities, and may be an alternative food source for higher predators. As a cold-water species, krill is particularly vulnerable to the warming of the Southern Ocean, particularly in conjunction with ocean acidification. Krill hatching success decreases with increasing CO2 levels in the water (Kawaguchi et al. 2013).

Zooplankton comprise species that build shells made of aragonite or calcite. An increase in the acidity of the Southern Ocean is likely to first affect these planktonic species. CO2-driven acidification reduces the availability of the carbonate ion that calcium carbonate (CaCO3) shell–making organisms require to build their shells. Consequently, many organisms now have thinner shells and reduced growth rates (Doney et al. 2009b). The rapid change in the acidity of the ocean is already affecting calcifying organisms—for example, the shells of planktonic organisms in the Foraminifera are now about one-third lighter than in pre-industrial times (Moy et al. 2009). However, the AAD-led Southern Ocean Continuous Plankton Recorder Survey has observed very large blooms of planktonic Foraminifera, especially in the southern summer of 2004–05, when they dominated the surface plankton (up to 80 per cent of abundance) through much of the Southern Ocean south of Africa to Australia (Takahashi K et al. 2010).

It is important to note that different species respond differently to environmental changes. Although some are likely to be affected adversely, others might benefit from the changes (Iglesias-Rodriguez 2008). For example, diatoms, a major group of algae, may benefit from some of the environmental changes, such as increased stratification of the water column (Constable et al. 2016). However, changes at the base of the food web, such as to other phytoplankton and zooplankton, can potentially radically change the dynamics of the Southern Ocean ecosystem, and it is still unclear whether (or how) higher-order organisms will be affected.

The benthic environment

The bottom of the Southern Ocean offers rich habitats on hard and soft substrata to many species, which grow much more slowly than their temperate counterparts. Both fixed and mobile species, including sponges, molluscs, starfish and worms, are highly diverse and abundant. Bryozoans (moss animals) are particularly diverse and are highly endemic (Brandt et al. 2007). Based on the outcomes of the Census of Antarctic Marine Life, CCAMLR proclaimed 2 vulnerable marine ecosystems to protect species assemblages and aid the conservation of biodiversity (SCAR 2010).

At depth, environmental conditions are stable, and species communities and assemblages do not appear to change much. A threat to the biodiversity of the benthos is iceberg grounding. Icebergs break off glacier snouts and ice shelves, and often get caught in currents that transport them away from their calving sites. In shallow water, icebergs can become grounded, which stirs up the sediment and crushes benthic fauna that is in the way. The damage caused by grounding icebergs tends to be local. So far, these grounding events appear to have contributed to species diversity in benthic communities by creating a patchwork of areas that are in different stages of recovery. However, an increased rate of iceberg calving may cause more frequent disturbances to benthic areas and not leave sufficient time for populations to recover. Fast-growing organisms are likely to have a better chance of resettling than slow-growing ones. In the long term, although the benthos may not remain scarred and unpopulated, its communities may change in their species composition, and some organisms are likely to be lost, at least locally.

The nearshore benthic environment

Adjacent to coastal ice-free areas are the shallow nearshore benthic environments, which are teeming with life (Clarke 2008). This environment is thought to be under threat from increasing human activities and changing environmental conditions in some areas (Clark et al. 2015b).

Terrestrial environments

Antarctica is almost entirely covered in permanent ice and snow, and permanent or seasonal ice-free areas of exposed rock are rare (21,700 km2 out of 14 million km2) (Bockheim 2015, Burton-Johnson et al. 2016). Ice-free areas are either isolated mountain tops, mountain ranges, dry valleys, exposed coastal fringes or offshore subantarctic islands.

Air temperatures vary with latitude and altitude. At coastal locations on subantarctic islands, average air temperatures are generally around 2–6 °C, with a range of less than 10 °C (Bergstrom et al. 2006a). Coastal average air temperatures around Australia’s Antarctic stations are milder than the interior and can rise to more than 0 °C in summer, but drop to less than –30 °C in winter. The region between 60°S and 70°S is the cloudiest on our planet, with 85–90 per cent cloud cover throughout the year (Bargagli 2005). Winds that are generated in the interior of the continent drive cold, dense air towards the coast. Smooth ice surfaces on the ice plateau and steep slopes at the coast reduce friction and intensify katabatic winds, which are strongest at the edge of the continent (often 180 kilometres per hour or more).

The main requisite for life in Antarctica is the availability of liquid water (Bergstrom et al. 2006b), which is mediated by solar radiation, temperature, and ice or snow cover (Convey et al. 2014). Most life occurs in ice-free areas. More than 99 per cent of Antarctica’s biodiversity is concentrated in areas that are permanently ice-free. In both the onshore and island realms, terrestrial habitat can be considered as islands or archipelagos surrounded by ice and/or sea, with most terrestrial biodiversity located near the coast (Frenot et al. 2005).

Within ice-free areas in Antarctica, terrestrial habitats include soft sediments (clays, sands, gravels), and habitats within, under and on top of exposed rock. In some areas, vegetation occurs where ancient penguin colonies used to be, or on extensive humic material derived from plants and built up over thousands of years. Visible life is mainly, although not completely, confined to lower altitude areas in coastal regions (Convey et al. 2014). In the subantarctic, many terrestrial ecosystems are built on extensive and often ancient peats, as well as on soil, gravel and rocks. Lakes and drainage systems are an important element of most ice-free areas of Antarctica and the subantarctic islands. A network of subglacial lakes also occurs across the Antarctic continent, and the lakes are likely to have biota in them (Kennicutt & Siegert 2011).

Most ice-free areas in Antarctica are young (less than 10,000 years old), but some ice-free refuges have been present for millions of years, allowing life to persist for multiple glacial cycles. Recent research has highlighted the role of volcanic areas in sustaining continuous conditions for life during past ice ages (Fraser et al. 2014). By using continent-wide biodiversity databases and ecological informatic approaches, recent biodiversity analyses have shown substantial spatial diversity across the continent, with 15 distinct ecoregions now recognised on the continent itself, and another 8 across the Southern Ocean islands (Terauds et al. 2012). Diversity within species, at local spatial scales of hundreds of metres to hundreds of kilometres, is also being discovered through phylogeographic approaches, reflecting the effects of both older glacial history and more recent events.

Higher vertebrates that use ice-free areas in Antarctica for nesting include Adélie penguins, and flying seabirds such as Antarctic petrels (Thalassoica antarctica) and snow petrels. Some seals use coastal Antarctic beaches as haul-out areas and fast ice (sea ice adjacent to land) for breeding. The subantarctic islands are major breeding and resting grounds for many species of penguin, flying seabirds (such as albatrosses and petrels), and fur and elephant seals. Most vertebrates, such as seabirds, penguins and seals, rely on the ocean for food.

Although the species richness of higher plants and insects is low in Antarctica, plants such as mosses and lichens are relatively well represented (Peat et al. 2007), as are invertebrates such as springtails, nematodes, tardigrades and mites (Velasco-Castrillon et al. 2014). Recent studies have also highlighted the diversity of microbial life in terrestrial Antarctica, with high-throughput DNA sequencing and metagenomic techniques (both used to analyse genomes) clearly showing that microbial diversity is much higher than previously thought (Fierer et al. 2012). The microbiotic communities (cyanobacteria, bacteria, fungi, viruses) are species-rich compared with communities elsewhere, and exist in streams, lakes, moss cushions and soil. Many microorganisms, such as some species of diatoms and cyanobacteria, are endemic to Antarctica (Vyverman et al. 2010). The environmental conditions that these species face across much of the continent are often described as some of the harshest on the planet. To survive these conditions, many of these species have developed unique physiological adaptations, including the ability to survive desiccation or freezing.

The subantarctic islands represent some of the rarest ecosystems on Earth. Subantarctic islands have a range of origins and ages, from remnant Gondwanan continental elements (South Georgia) to sea-floor material (Macquarie Island) that uplifted 600,000 years ago. There are at least 16 active volcanoes in the subantarctic and Antarctica, including (Bergstrom et al. 2006b):

  • Big Ben on Heard Island in the southern Indian Ocean
  • Mount Erebus and Mount Melbourne in the Transantarctic Mountains adjacent to the Ross Sea in Antarctica.

Terrestrial ecosystems are isolated from each other, and their floral and faunal communities are less complex than those at lower latitudes (Bergstrom & Chown 1999) or the Arctic region. For example, there are 900 species of vascular plants in the Arctic (Bliss 1971), compared with 2 species in the Antarctic (Komárková 1985).

Flora and invertebrate fauna are well developed on the subantarctic islands (see Box ANT6), with vegetation types ranging from tundra-like, sparse fellfields on the uplands to lush grasslands and herbfields on the coast. On the New Zealand subantarctic islands, lower-lying areas also have shrub, heath and coastal woodlands. Species diversity increases with decreasing latitude, but it is still lower in the subantarctic zone than in temperate regions, although species are often highly abundant. Compared with the terrestrial flora of Antarctica, subantarctic vascular plants are diverse and include tens of flowering plant species, including megaherbs and grasses. Mosses and liverworts are also a significant component of the landscape (Bergstrom & Chown 1999). Microbes, algae, fungi and lichens are also critical elements of subantarctic ecosystems. Trees and shrubs are absent from the Australian subantarctic islands, but do occur on other subantarctic islands. The faunal diversity is dominated by invertebrates and includes microarthropods, such as springtails and mites, and insects, including beetles and flies.

In Australia, Macquarie Island was the only breeding site for albatrosses or giant petrels where introduced species—rabbits, rats and mice—were present. An eradication program was successfully completed in 2012, and no rabbits or rodents have been sighted since. The populations of nontarget species most affected by the baiting under the eradication program appeared to recover well, except for skuas. They relied on rabbits as a major source of prey, which probably kept the skua population at a level well above its natural state. The vegetation is recovering quickly, and many seabird species are returning to the island to breed (see Box ANT1).

Long-term programs continue to monitor vegetation changes, and the abundance and distribution of nesting seabirds. To maintain the current status of Macquarie Island as free from introduced mammals, it is vital to implement rigorous biosecurity procedures. In 2013, new measures were introduced to facilities (such as the cargo facility at Hobart wharf) and to procedures. Both state and Australian government agencies are working together to achieve the highest level of biosecurity for transport of goods and people to Macquarie Island.

Klekociuk A, Wienecke B (2016). Antarctic environment: The living environment: Marine and terrestrial. In: Australia state of the environment 2016, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/theme/antarctic-environment/topic/2016/living-environment-marine-and-terrestrial, DOI 10.4226/94/58b65b2b307c0